Communication by technical means between a specific, finite number of subscribers is referred to as a communication session. A collaborative communication session is the process of communicating using a plurality of communication terminals (also referred to for short in the following text as terminals) for a single one of the communication ends. In other words, at least one of the communication ends (that is to say one of the subscribers) uses a plurality of terminals in order to communicate with at least one other communication end. A terminal at one communication end controls the collaborative session, for example by defining which communication media (for example audio, video, text, etc.) should be transmitted to which terminals in the collaborative session, and it therefore intrinsically has a so-called control role.
In order to allow the functions of controlling terminals in a collaborative communication session precisely for that controlling terminal (and in particular not to allow them for the non-controlling terminals), it is necessary to be able to allocate (or in other words: assign) the control role for one terminal in the communication system.
It is known from the document “3GPP Technical Specification TS23.237, V9.2.0, IP Multimedia Subsystem (IMS) Service Continuity; Stage 2” that collaborative communication sessions (collaborative sessions) can be carried out using a communication system which is designed on the basis of the Internet Protocol Multimedia Subsystem (IMS) of the 3GPP mobile radio standard.
In collaborative communication sessions in the IMS, one user with a plurality of terminals is registered with the IMS. The collaborative session is set up by one of the terminals requesting the IMS to transfer one of its communication media to another terminal of the user. This means that, after the media transfer, the medium is no longer received by that communication terminal but by the other communication terminal. In this case, the terminal requesting this media transfer is assigned the control role for the collaborative session.
Only one holder of the control role can subsequently control the collaborative session and, for example, carry out further media transfers. In the case of IMS collaborative communication sessions, the control role is always assigned to that terminal which requests the first media transfer.